Passover comes from the Bible, first mentioned in the book of Exodus. As God pronounced to the people of Israel enslaved in Egypt that he would free them, he said he would "Smite all the firstborn in the land of Egypt." However, he instructed the Israelites to put a sign of lamb's blood on their door posts: "and when I see the blood, I will pass over you." (Exodus 12) also see: Jewish Holidays - Names of Jewish Months

Chazeret

Bitter Vegetable (often lettuce)  is often used in addition to the maror as a bitter herb. The authorities are divided on the requirement of chazeret, so not all communities use it. Since the commandment (in Numbers 9:11) to eat the paschal lamb "with unleavened bread and bitter herbs" uses the plural ("bitter herbs") most seder plates have a place for chazeret. Some use a green vegetable (such as lettuce - as long as lettuce is not also used for karpas), some use the green top of a bitter herb, while some use a second bitter (such as raddish) for the chazeret.

Karpas

Vegetable (usually parsley) is dipped into salt water during the seder. The salt water represents the tears shed during Egyptian slavery. The dipping of a vegetable as an appetizer is said to date back to biblical times. It may now be identified with biblical descritpion of the Hebrew slaves marking their doorposts at the time of the first Passover. A bunch of hyssop was to be dipped in the blood of the paschal lamb and used to strike the lintel and the doorposts (Exodus 12:22) so that the tenth plague (death of the firstborn) would not be visited upon their households.

Maror 

Bitter Herbs (usually horseradish) symbolize the bitterness of Egyptian slavery. The maror is often dipped in charoset to reduce its sharpness. Maror is used in the seder because of the commandment (in Numbers 9:11) to eat the paschal lamb "with unleavened bread and bitter herbs." Some prefer mild horseradish at the seder; others say that it doesn't serve its purpose (to remind us of the bitterness of slavery) unless it's hot enough to bring tears to the eyes.

Pesach

Hebrew for the festival of Passover. The word pesach comes from a Hebrew root meaning "pass by" or "to spare." While the word "Pesach" appears in Hebrew on most seder plates, it is merely decorative. The word also refers to the Pesach (or Paschal) lamb which was sacrificed as a special offering in honor of the festival. The zeroa (shankbone) has its own place on the seder plate as a symbol of this sacrifice

Zeroa

The Shankbone is symbolic of the Paschal lamb offered as the Passover sacrifice in biblical times. In some communities, it is common to use a chicken neck in place of the shankbone. Vegetarian households often use beets for the shankbone on the seder plate. The red beets symbolize the blood of the Paschal lamb, which was used to mark the lintel and doorposts of the houses during the first Passover (Exodus 12:22)

Tips on how to prepare a Seder Plate